Add New Parameter to Existing URL using htaccess
PHP Snippet 1:
RewriteEngine on
RewriteCond %{QUERY_STRING} ^(?!email)
RewriteRule index3.php$ /index3.php?email=somevalue [L,R,QSA]
PHP Snippet 2:
RewriteCond %{THE_REQUEST} !\?email= [NC]
RewriteRule index3.php$ /index3.php?email=mail [L,R]